1. Field of the Invention
The present invention relates to an electrophotographic photoconductor used in image formation based on electrophotography, for example, in copiers, electrostatic printing, printers, facsimiles, and electrostatic recording. The present invention also relates to an image forming apparatus using the electrophotographic photoconductor and a process cartridge using the electrophotographic photoconductor.
2. Description of the Related Art
Information processing systems using electrophotography are making remarkable progress recently. Particularly, laser printers and digital multifunction processing machines, in which information is converted into digital signals and recorded by means of light, have been extremely improved in terms of printing quality and reliability. Further, technologies used in these laser printers and digital copiers are applied to laser printers and digital copiers capable of printing full-color images by merging with high-speed printing technologies. For the above reasons, photoconductors (otherwise referred to herein as a “photoconductor”) used for such laser printers and digital copiers are required both to produce high quality images and to have high durability.
Photoconductors (OPCs) using organic photoconductive materials are widely used for laser printers and digital copiers using electrophotography, due to their cost, manufacturability, and non-polluting properties. The organic photoconductors (OPCs) are generally classified into (1) organic photoconductors using a photoconductive resin typified by polyvinyl carbazole (PVK), (2) photoconductors using a charge-transfer complex typified by PVK-TNF (2,4,7-trinitrofluorenone), (3) pigment dispersion type photoconductors using a pigment typified by phthalocyanine-binder, and (4) function-separated photoconductors in which a charge generating layer containing a charge generating material is combined with a charge transporting layer containing a charge transporting material.
However, each photosensitive layer of the organic photoconductors (OPCs) is easily peeled off by repetitive use, and therefore the potential of the photoconductors tends to decrease and the photosensitivity tends to degrade. Further, a scratch on a surface of the photoconductor tends to cause background smear, and degradation in image density and quality of the resultant images. Therefore, improvement of abrasion resistance of the organic photoconductors has been an important subject. Further, improvement of durability of the organic photoconductors has become a more important subject recently, in accordance with speeding up of the printing speed and the reduction in diameter of the photoconductors accompanied by downsizing of an image forming apparatus.
As a method of improving the abrasion resistance of photoconductors, there are known methods of providing a surface protective layer as its outermost surface layer of a photoconductor and imparting lubricity to the outermost surface layer, hardening the outermost surface layer, or incorporating a filler into the outermost surface layer. However, another problem occurs when these methods are used to prevent the abrasion of the photoconductor. Specifically, oxidized gases, such as ozone and NOx arising due to use conditions or environment, adhere to the surface of the photosensitive layer and decrease the surface resistance thereof, resulting in a problem such as blurring of the resultant images. Of these methods, the method of incorporating a filler into a surface protective layer is one of the effective methods to impart high-durability to photoconductors (see Japanese Patent Application Laid-Open (JP-A) Nos. 53-133444, 55-157748, 57-30846, 2-4275, 4-281461, and 2000-66434).
However, when a filler is incorporated into the surface protective layer for the sake of imparting high-durability to the photoconductor, the residual potential is conspicuously increased due to increased electrical resistance. The increase in residual potential is greatly affected by an increase of electrical resistance caused by the incorporation of the filler and an increase of charge trap sites. Meanwhile, when a conductive filler is used in the surface protective layer, the outline of an image is blurred, i.e., so-called image blur is caused, resulting in a significant influence upon the image quality, although the electrical resistance is reduced and the influence on the increase in residual potential is relatively small.
It is known that when the method of incorporating a filler into the surface protective layer is employed in order to reduce abrasion of the photosensitive layer, ozone, NOx or other oxidative substances caused by the repetitive use of the photoconductor and the environment surrounding the photoconductor adsorb onto the surface protective layer, and the electrical resistance of the photoconductor decreases depending on the frequency of use and the use environment, causing a problem such as image flow (image blur), etc.
So far, such a problem has been avoided to some extent because the substances causing the blurred images are gradually scraped off in accordance with the abrasion of the surface protective layer. However, in order to comply with the above-described recent demand for higher sensitivity and durability of photoconductors, a new technique must be provided. In order to decrease the influence of substances causing the blurred images, a method of adding an additive such as an antioxidant in the surface protective layer is effective, however, the additive typically does not have photoconductivity, and therefore the addition of much amount thereof in the surface protective layer causes problems such as degradation of sensitivity and increase of residual potential of the resultant photoconductor.
To reduce the occurrence of image blurring, the photoconductor is preferred to have higher surface resistance. Meanwhile, to suppress the decrease in initial residual potential of the photoconductor and the increase in residual potential in repetitive use thereof, the surface of the photoconductor and the film itself are preferred to have lower resistance. For this reason, there is a trade-off relation therebetween, which makes it difficult to solve the problem.
The increase in residual potential which is often observed when a filler having high insulating property is used in a surface protective layer of a photoconductor leads to a high-electric potential at bright areas in the image forming apparatus, causing degradation of image density and gray-scale properties. To compensate for the high-electric potential at bright areas, there is a need to increase the electric potential at dark areas in the image forming apparatus. However, when the electric potential is increased at dark areas in an image forming apparatus, the intensity of the electric field is increased, leading to not only an image defect, such as background smear, but also the shortening of the longevity of the photoconductor.
For this reason, it is hard to use a filler having high insulation property. Thus, a filler having a low insulating property, which has relatively less influence on the residual potential, is usually used, and in order to prevent blurred images caused thereby, a method of mounting a drum heater for heating a photoconductor is used. Although the occurrence of image blurring can be reduced by heating the photoconductor, this poses a great obstacle to the downsizing of an image forming apparatus, reduction of power consumption, and shortening of the start-up time of an image forming apparatus.
Recently, full color image forming apparatuses each using a roller charging unit and enabling energy-saving operation with less ozone generation and enabling its compactness are most commonly used. However, in order to achieve image forming apparatuses having higher durability and enabling higher speed operation, a fresh look has been taken at non-contact charging units utilizing corona discharge, which have been conventionally used. However, the charging unit utilizing corona discharge cause a generation amount of discharge products (ozone, NOx, etc.) greater than the roller charging units cause. When a photoconductor containing a filler in the outermost surface layer is used in an attempt to achieve high durability, image blur is liable to occur.
Further, for the purpose of achieving high image quality, image forming apparatuses each equipped with a lubricant supplying unit that applies a lubricant to a surface of an electrophotographic photoconductor so as to reduce the friction coefficient have been proposed in which the image transfer rate is increased to thereby reduce the occurrence of character dropout and transfer nonuniformity of solid images. Such an image forming apparatus that employs a lubricant supplying unit has another advantage in that the abrasion loss of the photoconductor and the occurrence of photoconductor filming can be reduced, thereby realizing longer operation life of the photoconductor. In order to achieve further higher-durability of a photoconductor, use of a lubricant supplying unit in combination with a photoconductor containing a filler in its surface protective layer makes it possible to achieve much higher-durability than that of a photoconductor containing no filler in its surface protective layer, because the resistance to abrasion of the photoconductor caused by a variation of an amount of lubricant applied by the lubricant supplying unit and the scratch resistance of the photoconductor surface are more improved than those of the photoconductor containing no filler in its surface protective layer. In the meanwhile, the problem with image blur has been avoided to some extent because substances causing blurred images are gradually scraped off in accordance with the abrasion of the surface protective layer. However, in the image forming process where the photoconductor surface is hardly scraped off, in order to achieve further higher-durability of photoconductors, stability of electrostatic properties, i.e., suppression of an increase in residual potential, is required for, in addition to the image blur resistance and the abrasion resistance.
As mentioned above, in use of a photoconductor that contains a filler in its surface protective layer serving as the outermost surface thereof for the purpose of achieving higher-durability, an effective method whereby problems with the reduction in initial residual potential, the increase in residual potential due to repetitive use, and occurrence of image blurring can be solved all together has not yet been found out so far. Thus, in reality, the subject of providing high durability to photoconductors and providing high image quality still remains in the field of high-speed image forming apparatus.